Investigating effects of heating orientations on nucleate boiling heat transfer, bubble dynamics, and wall heat flux partition boiling model for pool boiling

Abstract

Studying the nucleate pool boiling is an important research in the thermal science. Therefore, the pool boiling experiments are performed in this paper to investigate the effects of heating orientations on the characteristics of boiling heat transfer and bubble dynamics. Based on the measured data, the boiling heat transfer capability increases as the inclined angle of heating surface is elevated. This enhanced effect is pronounced in the lower heat flux region and is insignificant in the higher heat flux region, which is also reported in the previous works. The characteristics of bubble growth, departure, sliding, and coalescence as well as the corresponding timings can be clearly revealed in the frames taken from the high-speed videos. Based on the measurement and observation, the merged bubble diameter and departure frequency increase with the increasing heating orientation. However, the active nucleate site density is measured to be independent of the inclined angles. The relationship of bubble dynamics with the wall superheat and the heating orientation can be obtained using the least-square-error regression method. With the input of these regressed correlations, the wall heat flux partitioning model can be assessed with the present measured boiling curves under various heating orientations. Good agreement reveals that the measured results and the corresponding regression equations provided herein can assist in validation of other boiling heat transfer models. This is one of main contributions from the present work in addition to providing additional data to understand the boiling characteristics of pool boiling under variable heating orientations.